Biaxially-oriented polypropylene films are widely used as dielectric films for capacitors by taking advantage of their high moisture resistance in addition to their superior electrical characteristics such as withstand voltage performance and low dielectric loss characteristics.
Polypropylene films for capacitors are preferably used for high-voltage capacitors as well as various types of switching power supplies, converters, inverters and other filters and smoothing capacitors, and the demand for increasingly thin films has increased in recent years due to an extremely strong demand for capacitors having reduced size and higher capacitance.
Moreover, propylene film capacitors are beginning to be widely used as smoothing capacitors in inverter power supply circuits that control the drive motors used in electric vehicles and hybrid vehicles and the like for which demand has been increasing in recent years.
The capacitors for the inverter power supply circuit used in these vehicles and the like must continue to operate stably (by maintaining electrostatic capacitance) while withstanding high direct current voltage for a long period of time and over a wide temperature range of −40° C. to 90° C. as well as retaining compact size, light weight and high capacitance.
Consequently, the capacitor dielectric films used are required to have high withstand voltage characteristics (improved dielectric breakdown voltage) that do not break down (dielectric breakdown) even when subjected to a high direct current voltage load at higher temperatures while still being ultra-thin at a thickness of 1 μm to 5 μm (high stretching performance), and capacitors composed of such films are required to improve long-term durability (minimization of time-based changes in electrostatic capacitance) without breaking down at higher temperatures or at higher voltages and even if continuously subjected to a load for longer periods of time.
Methods for improving the dielectric breakdown voltage of a film by controlling crystallinity and surface smoothing performance have long been proposed for improving withstand voltage characteristics. For example, Patent Document 1 discloses a capacitor composed of a highly stereoregular polypropylene resin that contains an antioxidant. In addition, Patent Document 2, for example, discloses a technology relating to a film, and a capacitor that uses that film which realizes high molten crystallization temperature (high crystallinity) and control of surface smoothing performance by using a polypropylene resin having high melt tension. However, since high stereoregularity and high crystallinity alone cause a decrease in stretchability and result in increased susceptibility to film breakdown in the stretching process, the technology of Patent Document 2 is unable to adequately correspond to the needs of the rapidly growing capacitor market due to being undesirable in terms of production.
On the other hand, in order to improve electrostatic capacitance in a capacitor of equal volume, it is necessary to reduce the thickness of the dielectric film. Although it is essential to improve the stretchability of the resin and cast rolled sheet in order to obtain an ultra-thin film in this manner as was previously described, improving this characteristic is a technique for improving withstand voltage, or in other words, is typically a property that is contradictory to improving crystallinity.
In contrast, Patent Document 3 discloses a finely surface-roughened film that is stretched from a cast roll having a comparatively low amount of β-type crystals using a resin having a balance between a specific range of molecular weight distribution and stereoregularity. This stretched, finely surface-roughened film is a thin film having withstand voltage characteristics, and although it is a finely surface-roughened film that has reached a level capable of satisfying the aforementioned three characteristics as a result of having a suitable degree of surface roughening, there is still room for improvement in order to satisfy severe requirements relating to long-term withstand voltage performance at high temperatures.
Moreover, Patent Document 4 discloses that both high withstand voltage performance and thinned fill can be realized without having to increase stereoregularity by adjusting molecular weight by containing a low molecular weight component. However, there are no examples or suggestions relating to long-term durability or withstand voltage performance required by the market, and thus cannot be said to be adequately satisfactory.
On the other hand, as is also disclosed in Patent Document 1, antioxidant is known to at least have some effect on long-term withstand voltage performance and capacitor electrical performance.
Patent Document 5 discloses a technology for inhibiting dielectric loss to a low level with a suitable combination and incorporated amounts of phenol-based antioxidants. However, there are no examples or suggestions regarding capacitor service life (or life performance (long-term durability)) when subjected to a high voltage load or long-term dielectric performance at high temperatures. More recently, Patent Document 6 discloses a technology for improving insulation resistance at high temperatures by using an antioxidant having a high melting point. However, there are no examples or suggestions regarding long-term withstand voltage performance at high temperatures or when subjected to a high voltage load in this document as well.
In this manner, the severe demand from the rapidly growing capacitor industry for long-term durability (capacitor life (service life) performance) when subjected to high-voltage loads at high temperatures has yet to be satisfied even with the aforementioned technologies.